docs(mssql): add comments and .env.example

- Add file header with usage instructions and Developer edition licence notice
- Document all environment variables with inline comments
- Reorganise MSSQL_PID edition list into Free / Development / Paid categories
- Add .env.example with documented defaults for SA password and edition

compose/sql-server-2025/.env.example

@@ -0,0 +1,23 @@
+# Microsoft SQL Server 2025 — environment variables
+# Copy this file to .env and adjust values for your environment.
+
+# SA (system administrator) password.
+# Must be at least 8 characters and include uppercase, lowercase, digits,
+# and/or special characters.
+# The default is for local development only — always change for production.
+MSSQL_SA_PASSWORD='1q2w3e!Q@W#E'
+
+# Product edition (default: EnterpriseDeveloper).
+# IMPORTANT: Developer editions are for local development and testing only.
+# They may NOT be used in production environments.
+# Free:
+#   Evaluation          — full feature set, production use allowed, 180-day limit
+#   Express             — production use allowed, resource-limited (CPU/memory/size)
+# Development only (no production use rights):
+#   EnterpriseDeveloper — default, full feature set
+#   StandardDeveloper
+# Paid:
+#   Standard
+#   Enterprise          — CPU core utilisation restricted to 20 physical / 40 hyperthreaded
+#   Enterprise Core     — CPU core utilisation up to operating system maximum
+MSSQL_PID=EnterpriseDeveloper